The present invention relates to electrical fault protective devices, and more particularly to exhaust control devices for expulsion-type fuses. The electrical fault protector of the invention is designed to protect power factor capacitor banks in power distribution systems where discharge of hot ionized gases and metallic materials from an expulsion fuse may damage the capacitors and cause internal bank flashover.
Expulsion fuses (also known as expulsion-fuse units) are vented fuse protection units having a body part and a fusible part or link and a leader, in which an overcurrent passing through the fusible element rapidly heats and melts the fusible element. The high current density opens the circuit (i.e., the fuse `blows`) and creates an arc. The expulsion effect of gases produced by the resulting arc on the lining of the fuse body rapidly discharges the fusible part (including combustion products) connected to the leader and extinguishes the arc. Expulsion fuses known in the prior art include single-ended and double-ended discharge types.
In the design of power factor correction capacitor banks, the present expulsion fuse art is limited to application on power systems with up to 6,000 amperes fault available. Expensive current limiting fuses are generally used on capacitor banks at fault currents above 6,000, but are quite costly. A need has thus developed to provide expulsion fuse protection in an economical and reliable way at interrupting ratings of up to 10,000 amperes and above. The prior art was unable to provide protection above 6,000 amperes with single-ended expulsion fuse designs. The expulsion discharge products and dynamic forces developed by single-ended fuses large enough to handle 10,000 amperes are known to damage capacitor block bus systems. Further, the capacitor bank discharge current known to occur when a capacitor fails in a large bank is sufficient to destroy the single-ended expulsion-type fuses. Also, the disintegrating fusible link and the hot ionized gases developed on high energy interruptions are blown into adjacent capacitors, causing internal capacitor block flashovers and resultant damage.
Double-ended expulsion-type fuses vent the fuse at both ends. Such a design achieves higher power--frequency fault protection and is capable of handling the capacitor bank discharge currents without damage.
U.S. Pat. No. 2,340,055 shows a protection device for telephone lines in which a chamber receives exhausted expulsion gases from a horizontal fuse bank located above the chamber. The expulsion gases are released from vent holes in the lower sides of the fuses through slots in a fuse supporting panel. The expulsion discharge occurs at the center of the fuses rather than at the ends of the fuses.
U.S Pat. Nos. 3,575,683 and 3,868,617 both show exhaust control muffler devices for use with a fuse. These devices are located axially from one end of the generally elongated fuse and are generally cylindrical in shape. They do not employ a common bus muffler configuration that provides for the successful operation of a plurality of fuses.